1. The documentation states that this boundary condition is based on solving DDt(psi, U) = 0 at the boundary.
2. The wave speed is calculated based on velocity normal to the boundary and the speed of sound (sqrt(gamma_/psi)).

To my understanding, the first usage of 'psi' refers to the field to which this boundary condition is applied whereas the second 'psi' denotes the compressibility.

The documentation is ambiguous and this could be solved simply by adding a note stating that the first usage of 'psi' refers to the field to which this boundary condition is applied.
Also, there is a space missing between "onsolving".

A similar note could be added advectiveFvPatchField.H, even though the compressibility is not used in this class.